Field of the Invention
The present invention relates to an electric motor having a low short-circuit torque that is of particular benefit for use in a drive device with a plurality of motors driving the same element.
Brief Discussion of the Related Art
A magnet motor generally comprises a rotor provided with magnets and a stator formed from a bundle of sheets surrounded by electric circuits in the form of coils forming electric phases allowing the motor to be controlled.
Some drive devices comprise a first motor and a second motor, which are connected to the same element so as to be able to both drive this element independently of one another. The rotors of the two motors may be connected to the same shaft for this purpose.
In a normal operating mode only the first motor is controlled so as to drive the element, the second motor being used only in the event of failure of the first motor.
This redundancy makes the operation of the drive device more reliable, to the detriment however of a relatively significant mass of the drive device.
In addition, some failures of the motor or of the control electronics thereof may cause the motor to exert a braking force. The motors therefore must be dimensioned so as to be able to compensate for this braking force produced by the faulty motor. In particular, when the faulty motor has a short-circuit between phases, the driving of the element by the other motor causes the rotation of the rotor of the faulty motor, which in turn induces in the short-circuit phases a current that cannot be eliminated and results in the appearance of a resisting torque.
The motors therefore must be dimensioned so as to be able to displace individually the element whilst taking into account not only external stresses exerted on the element, but also the resisting torque that would be produced by a faulty motor. The motors therefore must be able to produce a greater torque, which increases the cost, bulk and mass of said motors.
It is recalled that the maximum short-circuit torque CCC has a value of CCC=3/2·K2/(2p·(L−M)) and appears at the speed V=R/(p·(L−M) with a gradient p=3/2·K2/R, in which K is the coefficient of phase/neutral torque, p is the number of pairs of poles, L is the inductance of the motor, M is the mutual phase/neutral inductance, and R is the phase/neutral resistance.
In order to limit this torque it is known to increase the resistance of the coils in order to increase the speed of rotation of the rotor at which the maximum torque appears. This solution is therefore only conceivable for motors functioning at low speed. This also increases the losses by Joule effect, degrading the performance of the motor and necessitating a cooling of the motor.